Hazy polyvinyl fluoride film and process



March 12, 1963 D. T. BOTTORF ETAL 3,081,208

HAZY POLYVINYL FLUORIDE FILM AND PROCESS 2 Sheets-Sheet 1 Filed Feb. 17, 1960 FIG awe/M0113 F R O T T 0 T H BC E H HE TE DL s E mm DJ March 12, 1963 D. T. BOTTORF ET AL HAZY POLYVINYL FLUORIDE FILM AND PROCESS Filed Feb. 17, 1960 2 Sheets-Sheet 2 SINGLE muse, rum: conrosmoa POLYVINYL FLUORIUE nnvmc A MELT-FLOW NUMBER (5) or 5-10 m A man SOLVENT STARTING IATERTAL EXTRUDE AS FILI AT ELEVATED TEMPERATURE -FIRST STEP COOL FILI T0 TEMPERATURE BELOW [00C secono 51,5?

0PTIONAL STEP DRY THE FILM FINAL STEP INVENTORS .DONA'LD THOMAS BOTTORF JAMES LEE HECHT VIRGIL EUGENE JAMES ATTORNEY 3,081,208 HAZY POLYVINYL FLUORIDE FILM AND PROCESS Donald Thomas Bottorf, Tonawanda, and James Lee Hecht and Virgil Eugene James, Buffalo, N.Y., assig iors to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Feb. 17, 1960, Ser. No. 15,003 8 Claims. (Cl. 154-46) This invention relates to the preparation of synthetic organic polymeric films. More particularly, it relates to the preparation of strong, durable films of polyvinyl fluoride having a satin finish.

Strong films having reduced specular reflectance and low transparency, hereinafter called a satin finish, are in great demand. They are used in laminates, screens, shower curtains and in a host of other decorative and protective uses. However, the most successful prior art processes for forming such films leave much to be desired.

3,031,203 Patented Mar. 12, ldfiB 7 If the film is not elongated, the equation reduces to The processes heretofore known involve laminating thin metallic or weak hazy polymeric films to a strong polymeric film base or depositing particles or similar additives in or on the base film. In any case, the resulting product suffers from problems due either to poor adhesion to the base film or to the tendency of additives to exude to the surface or to the tendency of particlecoated surfaces to be abraded easily. Furthermore, the elasticity of any resulting laminated or coated or addifive-containing film is limited by the surface laminate, the coating or the additive.

It is an object of the present invention to produce a self-supporting polymeric film having a satin finish without using additional materials of any kind. Another object is to produce a novel strong polyvinyl fluoride film having a satin finish. Other objects will appear hereinafter.

The product of the present invention is a polyvinyl fluoride film characterized by a surface exhibiting a reticular structure having insular areas of random size and random shape, the number of insular areas enclosed within the reticular structure ranging from about 5 to about 40 per 0.01 square millimeter of the surface of the film, the reticular structure penetrating the surface of the film to a depth of a few microns, the film being further characterized by a haze level of at least as measured in accordance with ASTM method Dl003-52.

In the drawing, FIGURE 1 is a photomicrograph of the novel polyvinyl fluoride film of this invention when Viewed with transmitted light where one surface of the film is in sharp focus at a magnification of 186x;

FIGURE 2 is a photomicrograph of the novel film as in FIGURE 1 wherein the magnification is 22X;

FIGURE 3 is a photomicrograph of the novel polyvinyl fluoride film of this invention viewed with trans mitted light when the microscope is focused on an edge of the film at a magnification of 258x. This profile view shows that the surface irregularities, as measured from lov/s" to highs, are the order of 1l0 microns;

and

FIGURE 4 is a how sheet of the process of this invention.

The object, preparing a stron polyvinyl fluoride film having a satin finish, is accomplished by extruding a coalesced, single phase, fluid composition containing between a minimum percentage, P, and about 60% of polyvinyl fluoride in a latent solvent for the polyvinyl fluoride, the polyvinyl fluoride having a melt-fiow-number of 5-l0, in the form of a film at an elevated temperature sufiicient to maintain the fluid composition in a single phase; immediately thereafter, cooling the film to a temperature below about 100 0, preferably below 25 C.,

. P=26.5(l.06) The term latent solvent as used herein is defined as an organic liquid, chemically inert with respect to polyvinyl fluoride, and having no significant solvent or swelling action on polyvinyl fluoride at room temperature, but at an elevated temperature below its normal boiling point being capable of solvent action sufiicient to cause polyvinyl fluoride particles to coalesce.

The latent solvent should be a liquid at room temperature having a normal boiling point greater than C.

.It should be thermally stable at least up to the temperatures to which it will be subjected during the process of this invention and preferably up to its normal boiling point. It should not react chemically with either the polymer or the materials of construction of the process equipment over the expected temperature range. The preferred latent solvents for use in the process of this invention are selected from the group consisting of gamma butyrolactone, N,N dimethylacetamide, tetra- ,rnethylene sulfoue, dimethyl-sulfolane, dimethylsulfoxide,

N,N-dimethylformamide, N-methyl-2-py-rrolidone and gamma-valerolactone. Other lactams besides N-methyl- 2-pyrrolidone and other lactones besides gamma-Valeralactone are also useful.

To produce the film of the present invention, it is necessary to employ in the process orientable polyvinyl fluoride of high molecular weight. Films made fro-m polymer ranging in melt-flow number from approximately 5 up to 10 may be used. The melt-flow number of a particular polyvinyl fluoride is the square of the average diameter in inches of a roughly circular film disc resulting from the pressing between two polished chromium-plated steel plates of a l-inch diameter wafer consisting of 1.00:0.01 gram of said polymer in dried, particulate, compressed form for 5 minutes at 260:1 C. under a total load of 12,250 pounds. Specifically, the procedure involves drying a quantity of particulate polyvinyl fluoride (LOO-0.01 gram) to less than 0.2% Water by weight; transferring it to the 1-inch diameter die of a Buehler metallurgical mounting press and pressing the die for a few moments at a load of about 5000 pounds. After releasing the load, removing and disassembling the die, the resulting polymer Wafer is an inch in diameter and about 100 mils thick. The polymer wafer is centered between two polished chromium-plated steel plates, 0.020 inch thick and cut to 5 inches by 8 inches with corners and edges smooth. This assembly is then centered between the platens of a Carver laboratory press, the temperature of the surface of the center of each platen being maintained at 260:1" C. The Carver press has 5-inch by S-inch electrically heated platens and is rated for a load of up to 10 tons. The polymer wafer is then pressed for 5 minutes at this temperature under a load of 12,250 pounds as indicated by the load gauge pointer. As the polymer mass melts and increases in diameter, it is necessary to pump up the press periodically to hold the load constant. At the end of the 5-minute pressing period, the load is immediately released and the plate-polymer-plate assembly is removed from the press and immersed quickly in cool water. After allowing the assembly to remain for several minutes under Water, the plates are separated and the film disc removed, dried by blotting and its diameter measured to the nearest 0.01 inch. If the film disc is irregular, eight diameters are measured and averaged arithmetically. The square of this diameter is the melt-flow number of the polymer.

The polyvinyl fluoride-latent solvent, single phase, fiuid composition suitable for the purpose of this invention may be prepared by any convenient expedient. In general, a proportioned mixture of particles of high molecular weight, orientable polyvinyl fluoride and latent solvent, the proportions selected in accordance with the previously given equation is heated until the particles coalesce to form the single phase fluid composition. As shown in the equation, the selection of the particular latent solvent/ polymer proportions will depend upon the melt-flow number of the polymer and the stretch ratio product of the subsequently obtained film. The minimum percentage of polymer that can be used is about 34%, i.e., where no stretching step is employed (the stretch ratio product is 1), and the polymer has a melt-flow number of 5. As the melt-flow number of the polymer increases, the mini mum polymer content that must be used in the feed mixture to produce a satin finish film increases. For example, when a polyvinyl fluoride exhibiting a melt-flow number of about is used and no'stretching is provided, the minimum percentage of polymer rises to about 46%.

Stretching the latent solvent-containing polyvinyl fluoride film produced by extrusion and subsequent quenching tends somewhat to reduce the haze level of the film and thus to decrease the satin finish of the film. To compensate for this reduction, the minimum percentage of polymer used in the initial fluid composition must be increased in accordance with the equation as a function of the stretch ratio product. The stretch ratio product is obtained by multiplying the stretch ratio for the first direc tion of stretch by the stretch ratio for the second and mutually perpendicular direction of stretch. The stretch ratio for either direction is obtained by dividing the stretched length of the film by its original length, the lengths being measured in the direction of stretch. Thus, where no stretch is applied the stretch ratio is unity.

There is no aesthetic objection to films of exceedingly high haze levels. The upper limit on polymer content in the original feed composition is dictated by operability requirements in the process. For example, as the polymer content in the feed mixture increases, the power input requirement for a particular throughput is increased to the point where the work/heat input of the extruder may result in thermal degradation of the polymer. A percentage of about 60% polymer in the feed mixture with the latent solvent is a practical upper limit.

The single phase fluid composition is extruded in the form of a film by conventional means at a temperature suflicient to maintain the fluid composition in a single phase. This temperature may vary anywhere from about 120 C. to about 250 C., at which temperature degradation of the polymer occurs rather rapidly. The more realistic upper limit for this temperature is the temperature above which the latent solvent tends to produce excessive vapor, such boil-oif manifesting itself as bubbles and other undesirable defects in the film. When using gammabutyrolactone as the latent solvent, an extrusion temperature of about 150 C. to about 180 C. is preferably employed. When N,N-dimethylacetamide is used, the extrusion step is preferably carried out in a range of about 140 C.-150 C.

The next step in the process, the step of rapidly cooling or quenching the extrudate from the casting hopper is most easily accomplished by directing the extrudate (the film) into a Water bath maintained at the quench temperature. This temperature must be below 100 C. and is preferably below 25 C.

The next step, the stretching step, is preferred in order to obtain a strong durable polyvinyl fluoride film having a satin finish. Although orientable polyvinyl fluoride film containing latent solvent emerging from the quenching step can be successfully stretched either in one direction or in two mutual-1y perpendicular directions at a temperature as low as room temperature, the preferred operating range for the continuous process lies between 30 C. and 185 C. Temperatures in this range are sulficient to maintain adequate friction between the solvent-containing film and the rolls employed to stretch the film longitudinally. Specifically, within this range, it is preferred that the first direction elongation be performed between 30 C. and C., elongation in the second direction mutually perpendicular being preferably performed between 60 C. and 185 C.

ctually, solvent-containing orientable polyvinyl fluoride film may be successfully stretched at temperatures up to that above which it begins to adhere to the surfaces of the process equipment or above which it is no longer sufficiently self-supporting. This will occur in the vicinity of the melting temperature range for the solvent-containing film, which range will vary somewhat with the molecular weight of the polymer but more particularly with the solvent content of the film. However, to minimize the heat input requirements to the process and to reduce the possibility of thermally degrading the polymer, it is desirable to avoid operation at such high temperatures.

The process of this invention may be carried out to elongate a latent solvent-containing polyvinyl fluoride film by stretching or drawing techniques common in the art. It should be understood that, for the purpose of this invention, the terms stretching and drawing are intended to embrace the technique of expanding a tubular polymeric structure by fluid pressure. When film is to be elongated in two mutually perpendicular directions according to the process of this invention, elongation in these two directions may be accomplished either simultaneously or sequentially by techniques well known in the art.

Preferably, the process is carried out by continuously stretching the latent solvent-containing polyvinyl fluoride film sequentially in each of two mutually perpendicular directions. This may be done either by first stretching in a longitudinal direction followed by stretching in the transverse direction, or by reversing the sequence of stretching, since advantages accrue to either sequence.

A major portion, and preferably substantially all of the solvent remaining in the film after elongation, is removed in a final drying operation; for example, subjecting the elongated film in air to a temperature and for a time sufficient to volatilize the major portion of the remaining latent solvent from the film down to a concentration of not more than about 0.2% by weight latent solvent in the film. However, if the temperature of the solvent-free film is allowed to exceed about 0., much orientation may be lost if the film is not restrained. With film properly restrained, a stream of air at a temperature in the vicinity of 200 C. may be employed for volatilizing latent solvent from biaxially elongated, oriented film.

The polyvinyl fluoride films produced by the process of this invention scatter and diffuse light to such a degree that they exhibit not only a low level of see-through transparency but also a markedly reduced specular reflectance. To the eye they exhibit a softly textured satin finish while yet retaining a smooth flatness to the touch. They. exhibit a haze level of at least 30%. The percent haze of film samples is measured in accordance with the standard method of test for haze and luminous transmittance of transparent plastics, ASTM designation: D1003- 52. The apparatus employed in carrying out these measurements is a Model AUl0a Complete Pivotable-Sphere Hazemeter with special 4-cell high sensitivity exposure head and automatic photomeric unit, manufactured by Henry A. Gardner, Laboratory, Inc., Bethesda, Maryland. Several determinations of haze level are made for each film sample and the arithmetic average is recorded. In general, samples from a given film vary in haze level less than about 2% from the arithmetic average.

Microscopic examination of the surfaces of these novel proximately 33 *mils, into a water bath at 23 C. to form a gamma-butyrolactone-containing polyvinyl fluoride film.

The film was passed from the water bath into a stretching apparatus where it was stretched to a stretch ratio polyvinyl fluoride films, as shown in FIGURES 1-3, re- 5 product of 4.0. Stretching was accomplished by elongatveals a somewhat netlike or recticular pattern. Employing the film 100%, i.e., 2x, longitudinally over a bank of ing transmitted light at magnifications in the range of horizontally mounted, mutually parallel heated idler rolls l75-200x, where the depth of field is small, it is readily maintained at about 70 C. followed by stretching the determined that the pattern does not exist throughout the film 100%, i.e., 2X, transversely in a tenter frame at an entire thickness of the film but only to a depth of a few 0 ambient temperature in the vicinity of 120 C. microns. Thereafter, the film was dried by subjecting the film These films may be used in applications where light while under tension to restrict any dimensional change transmission without transparency is desired, as for ex- :0 a stream of air at a temperature of about 190 C. for ample in certain glazing applications, the construction of about 120 seconds. I air-supported structures, room dividers, partitions and The final was 4.0 mils thicli, had a satin finish and movable screens, shower curtains, lamp shades, patio displayed a reticular structure 111 its surface having about screens and skylighting, etc. They may be laminated to 540 insular ar as P Square millimetef- When such substrates as aluminum, masonite, cement-asbestos tested for haze level, the film displayed 39% haze. It boards, cellulosic hardboards, plywood, etc., said laminad an average tensile strength of 12,000 ps1. and an tions to be employed in exterior house sidings and inaverage elongation of 140%. dustrial building sidings such as spandrel panels in curtain In?! Control Whefem the above described p wall construction. They may be aluminized by vapor W Identical Q P the 115% of 30 p y deposition techniques in vacuo and then the aluminized l feed mlxtum (Whereas the FQW q a surface laminated to post-formable steels for fabricamlnlmllm the final 4.0 mil thick film fell outtion into such things as automobile wheel covers and a h qmgemehnts gift g P 51 l l- 1 g 7 1110i soft chrome trim members. A lamination of an alu- We a Satin 1113 an lts eve Was y ts minized satin finish polyvinyl fluoride film to a heavier tensile gf l elongatlon were substantially the vinyl film and thence to an automobile hard-top may Same e eXamP simulate the efiect of brushed stainless steel. These poly- XA P 2 4 vinyl fluoride films may be employed in laminar con- I th 1 b t 1 t d 1 structions such as counter and table tops, Venetian blind fl i fi gy one Po slats, leatherette type upholstery fabrics, interior Wall y. ewge i e as m Xampe 1 to Orm con}- finishes, automobile seat covers, door paneling and head F Q gi fi g P q g liners, etc. In the manufacture of fiber-reinforced polya e 3 me Ow er of 18 p0 ymer is so meric resin shaped structures, these novel polyvinyl fiuo- 35 g i f d t {t d t t f ride films may be employed advantageously as mold re- 100 g Ti ygg f e 0 2 a i 2 lease sheets during the curing step, thereby imparting a f I a 1 g i s e i rich satiny appearance to the finished structure. 0 a 3 2 3 5 ga .2 s The following illustrative examples constitute specific P 51 i W X H e r a e embodim ns of the recess of this invention and are not the hps bemg set at an Openmg approximately 20 mlls' f d E 1 th" b t The composition was extruded into a water bath at 20 C. 3 0 8 mm a replesfin to form the latent solvent-containing polyvinyyl fluoride mode contemplated for carrying out the present invention. fihm ln the examples tensile strength in lbs/square inch is The film was stretched and dried in the a g 2 3 ZE-E es; f a s; scr iibell-ftir Example 1 to the stretch ratio products shown an is eermie nngn i a arae i per minute until the sample breaks. The tensile The resulting film all displayed a atin fini hth ir strength is determined in each of the two mutually persurfaces showed a reticular structure of between 54() pendicular directions of stretch and the average is preinsular areas per 0.01 square millimeter and the haze sented. Percent elongation is the percent increase in levels were all above 30% as shown in Table I.

Table l Stretch Ratios Average Average Average Percent Melt; Stretch Haze Tensile Elonga- Example Polymer Flow Ratio Level Strength tion in Feed Number First Second Product (percent) (p.s.i.) (percent) Direction Direction length of the film at the breaking point in each direction EXAMPLES 5-10 and an average 13 presgmgd' In these examples, gamma-butyrolactone and polyfi vinyl fluoride were blended as in Example 1 to form com- G b 1 t (1 6d th 1 lfl 65 $3M? co ir'gaining1 11156 percentige o; pglymer; shown in animal lltYlOrEiC one was .en Wl' p0 yvmy uoa e e me tow num er 0 t is porymer was ride having a melt-how number of 9.8 in 21 Hobart mixer 7,4 -7,6 Ea h mixt w fed i t an t de t a to f rm a mix r containing 51% of the p ymer an rate of 90-95 lbs/hour where it was heated to a tempera;- 49% of the solvent. The mixture was fed into an exture of C. C. to form the fiuid composition. truder at a rate of 30 lbs./ hour where it was heated to a 70 Each composition was extruded through a 14"-wide hoptemperature of 155 C. to completely coalesce the polyper, the lips being set at an opening of approximately 50 vinyl fluoride particles and to form a single phase fluid rmls. The compositions was extruded into a water bath composit on with the gamma-butyrolactone. at 15 C. to form the latent solvent-containing poly- The composition was extruded through the lips of ,an vinyl fluoride film. I

8"-wide hopper, the lips being set at an opening of ap- 75 The. film was stretched and dried in the manner de- 7 scribed for Example 1 to the stretch ratio products shown in Table II.

The resulting films all displayed a satin finish; their surfaces showed a reticular structure of between 5-40 insular areas per 0.01 square millimeter and the haze levels while evolving solvent therefrom; and drying said film; the minimum percentage, P, being determined from the equation:

b we ll b e 30% a ho n in Tabl 11, wherein F is the melt-flow number of said polyvinyl In a control, wherein the above-described procedure fluoride, and S is the stretch ratio product. was identical except for the use of only 40% polymer in 3- A process comprising the steps, in sequence, of exthe feed mixture (whereas the equation requires a miniu g a COaleSCed, Slfigle P fluid comp ition Conmum of 44%), the final film fell outside the require- 10 taming polyvinyl fluoride and a latent solvent for said merits of the present invention. It did not have a satin polyvinyl fluoride in the form of a film at an elevated finish and its haze level was only 16%. temperature suflicient to maintain the fluid composition in Table II Stretch Ratios Average Average Average Percent Stretch Haze Tensile Elonga- Example Polymer Ratio Level Strength tion in Feed First Second Product (percent) (p.s.i.) (percent) Direction Direction 50 1. 55 1. ea. 02 s 11, 500 175 45 1.8 2. 4. 4. 32 49 15,100 135 50 1. '1 2. 1 s. 57 e3 12, 700 180 4? 1. 7 2. a 1. 0 54 14, can 150 50 3.0 3. 3 9. 9 a1 17, 900 95 4s 2. 5 a. 2 8.0 32 11, 200 10c 10 1. s 2. 2 3. so 16 10, 450 140 EXAMPLES ll AND 12 a single phase, said polyvinyl fluoride having a melt-flow In these examples, N,N-dimethylacetamide was blended nu'ntlber E fg qg r 18mg fi a i f with polyvinyl fluoride having a melt-flow number of is g 1 ggg gfl TQ E i 6.76.9 to form compositions containing the percentages a empera um e i nng Sm film i two mutually perpendicular directions to a stretch ratio of the polymer shown in Table III. roduct of 3 1O hu 1 1 th f d Each mixture was fed into an extruder at a rate of S in sad a; v e Y vmg so vent g g i an 100-120 lbs/hour where it was heated to a temperature terymiied e f f percentage emg of 146 C.-148 C. to form the fluid composition. Each Om e aqua composition was extruded through a 27"-wide hopper, the lips being set at an opening of approximately 20 mils. wherein F is the melt-flow number of said polyvinyl The composition was extruded into a water bath at 15 fluoride, and S is the stretch ratio product. C. to form the latent solventcontaining polyvinyl fluoride 4. A process comprising the steps, in sequence, of exfilm. 4O trudlng a coalesced, single phase, fluid composition con- The film was stretched and dried in the manner detaming polyvinyl fluoride and a latent solvent for said scribed for Example 1 to the stretch ratio products shown polyvinyl fluoride in the form of a film at an elevated in Table III. temperature suflicient to maintain the fluid composition in The resulting films all displayed a satin finish; their sura single phase, said polyvinyl fluoride having a melt-flow faces showed a reticular structure of between 5-40 insular number of 5-10 and comprising from at least a minimum areas per 0.01 square millimeter and the haze levels were percentage, P, to about 60 percent of said fluid composiall above 30% as shown inTable III. tlon; cooling said film to a temperature below 100 C.;

Table III Stretch Ratios Average Average Average Percent Stretch Haze Tensile Elonga- Example Polymer Ratio Level Strength tion in Feed First .Second Product. (percent) (p.s.i.) (percent) Direction Direction Having fully disclosed the invention, whatis claimed is: and drying said film; the minimum percentage, P, being 1. A strong, durable, oriented polyvinyl fluoride film determined from the equation: having a smooth surface characterized by a reticular P=(26.5)(1,()6 r' structure said reticular structure having insular areas of Wherei r n F is th elrandom size and random shape, the number of insular fluoride e m number of Said Polyvinyl areas m 531d retlcular .rangmg j abom. 5 5. A process comprising the steps, in sequence, of exto about 40 per 0.01 square milhmeter, said film being f th h ct d b haz level of at least 307 tiu mg a coalesced, single phase, fluid composition con- 2 Z am g Ste Se Hence e taming polyvinyl fluoride and a latent solvent for said a proceis c f i a? q polyvinyl fluoride in the form of a film at an elevated a i fi 6 P cenlqposl 2; temperature suflicient to maintain the fluid composition talmgg P0 Y y F 6 an a atent so vent or Sal in a single phase, said polyvinyl fluoride having a melt- 1 Y} fiuofld? 111 the Q a at an l f flow number of 5-10 and comprising from at least :1 1 atllfe sufficfent to Qlallltam h fluldcomposltlon 111 minimum percentage, P, to about 60 percent of said fluid a Smgle Phase, Sald p y l' having a'melt-fiow composition; cooling said film to a temperature below number of 5-10 and comprising atleast a minimum per- 100 C.; elongating said film in at least one direction to a centage, P, of said third composition; cooling said film stretch ratio product of up to 10 while evolving solvent to a temperature below 100 C.; elongating said film in at therefrom; and drying said film; the minimum percentage, least one direction to a stretch ratio product of 3 to 10 P, being determined from the equation:

wherein F is the melt-flow number of said polyvinyl fluoride, and S is the stretch ratio product.

6. A process comprising the steps, in sequence, of extruding a coalesced, single phase, fluid composition containing polyvinyl fluoride and gamma-butyrolactone in the form of a film at a temperature of about ISO-180 C., said polyvinyl fluoride having a melt-flow number of -10 and comprising from at least a minimum percentage, P, to about 60 percent of said fluid composition; cooling said film to a temperature below about 100 C.; stretching said film in at least one direction to a stretch ratio product of up to while evolving gamma-butyrolactone therefrom; and drying said film; the minimum percentage, P, being determined from the equation:

wherein F is the melt-flow number of said polyvinyl fluoride, and S is the stretch ratio product.

7. A process comprising the steps, in sequence, of extruding a coalesced, single phase, fluid composition containing polyvinyl fluoride and N,N-dimethylacetamide in the form of a film at a temperature of about 140-150" C., said polyvinyl fluoride having a melt-flow number of 5-10 and comprising from at least a minimum percentage, P, to about 60 percent of said fluid composition; cooling said film to a temperature below about 100 C.; stretching said film in at least one direction to a stretch ratio product of up to 10 while evolving N,N-dimethylacetamide therefrom; and drying said film; the minimum percentage, P, being determined from the equation:

wherein F is the melt-flow number of said polyvinyl fluoride, and S is the stretch ratio product.

8. A process comprising the steps, in sequence, of extruding a coalesced, single phase, fluid composition containing polyvinyl fluoride and a latent solvent for said polyvinyl fluoride in the form of a film at an elevated temperature sufficint to maintain the fluid composition in a single phase, said polyvinyl fluoride having a melt-flow number of 5-10 and comprising at least a minimum percentage, P, of said fluid composition; cooling said film to a temperature below 25 C.; elongating said film in at least one direction to a stretch ratio product of up to 10 while evolving solvent therefrom; and drying said film; the minimum percentage, P, being determined from the equation:

wherein F is the melt-flow number of said polyvinyl fluoride, and S is the stretch ratio product.

References Cited in the file of this patent UNITED STATES PATENTS 2,417,293 DAlelio Mar. 11, 1947 2,721,114 Downing et a1 Oct. 18, 1955 2,748,042 Borgese May 29, 1956 2,791,000 Bechtold May 7, 1957 2,822,237 Dwamae Feb. 4, 1958 2,824,780 Sutterthwaite Feb. 25, 1958 2,914,436 Nakielny Nov. 24, 1959 2,953,818 Barton Sept. 27, 1960 

1. A STRONG DURABLE, ORIENTED POLYVINYL FLUORIDE FILM HAVING A SMOOTH SURFACE CHARACTERIZED BY A RETICULAR STRUCTURE, SAID RECTICULAR STRUCTURE HAVING INSULAR AREAS OF RANDOM SIZE AND RANDOM SHAPE, THE NUMBER OF INSULAR AREAS IN SAID RETICULAR STRUCTURE RANGING FROM ABOUT 5 TO ABOUT 40 PER 0.01 SQUARE MILLIMETER, SAID FILM BEING FURTHER CHARACTERIZED BY A HAZE LEVEL OF AT LEAST 30%. 